In a traditional cellular telecommunications system, the coverage of a cell is defined by the geographical area where Radio Frequency (RF) signals transmitted from a base station to a UE, and vice versa, can be successfully received and decoded. The “RF signals” may be more simply referred to herein as “radio signals.” The base station may be equipped with an antenna or antenna array that transmits and receives radio signals according to an antenna beam pattern that typically spans a quite large angle in azimuth and/or elevation. The wider the angle is, the lower the antenna gain becomes. Hence, there is a tradeoff between angular coverage and coverage range for a given antenna pattern. In order to have a large angular coverage in combination with high antenna gain, a steerable antenna array can be used to form and steer beams in desirable directions.
In the coverage-related discussion herein, a “cell” and its associated base station such as, for example, an evolved Node B (eNB or eNodeB), or a base station and its antenna array, may be referred to in an interchangeable manner and identified using the same reference numeral for ease of discussion. For example, a UE may be interchangeably referred to as receiving radio signals from a cell or an eNB, or the UE may be interchangeably referred to as receiving signals from a base station or the base station's antenna array.
FIGS. 1A-1E illustrate different examples of antenna beam patterns and their coverage range. In FIG. 1A, a base station antenna array 20 is shown providing an antenna beam pattern 26. Three UEs 22-24 are also shown in FIG. 1A as being physically present and operating (or registered) within the cell (not shown) associated with the base station 20. For the sake of discussion herein, the UEs 22-24 may be considered “attached” to or under the operative control of the base station 20. As shown in FIG. 1A, the antenna pattern 26 covers a wide angle with a limited range in the sense that only two of the three UEs—here UEs 22 and 24—receive radio coverage from the wide angle beam pattern 26. On the other hand, in FIG. 1B, the antenna array 20 is shown to provide another beam pattern 28. For ease of discussion, the same reference numerals are used in FIGS. 1A-1D to refer to the same entities. However, it is understood that, in practice, all of the beam patterns shown in FIGS. 1A-1D may not be necessarily provided by the same base station antenna; different base stations may provide different types of antenna patterns. Referring again to FIG. 1B, it is observed that although the antenna pattern 28 provides a greater range—which now provides radio coverage to the UE 23, the beam pattern 28 covers a narrower angle than the beam pattern 26. As a result, the UEs 22 and 24 may fall outside of the coverage area.
To provide coverage to all UEs 22-24, the antenna array 20 may be configured as a steerable antenna array as shown in FIG. 1C. The steerable antenna array 20 can provide individual antenna beams 30-32, which may be provided simultaneously or scanned through in time domain (as discussed later with reference to FIG. 1D). The multiple beams 30-32 resulting from the steerable antenna array 20 may not only effectively cover the wide angle of FIG. 1A, but also provide the range of FIG. 1B, thereby providing radio coverage to all three UEs 22-24 as shown.
The base stations in modern cellular systems may also employ beamforming in addition to the beam steering illustrated in FIG. 1C. Beamforming or spatial filtering is a signal-processing technique used in antenna arrays for directional signal transmission or reception. It is understood that digital content may be transmitted using analog radio signals. In beam-forming, the analog radio signals may be processed/shaped such that signals at particular angles experience constructive interference, while others experience destructive interference. Such analog beamforming can be used at both the transmitting and receiving ends to achieve spatial selectivity, such as, for example, rejection of unwanted signals from specific directions. The spatial selectivity may provide improved reception/transmission of signals in the system. Thus, beamforming can help improve wireless bandwidth utilization, and it can also increase a wireless network's range. This, in turn, can improve video streaming, voice quality, and other bandwidth- and latency-sensitive transmissions.
For a beam-forming system that only supports a set of fixed beams, all signals may be beamformed although the desired direction of transmission may be unknown or only known to some extent. Furthermore, some beamforming systems, such as, for example, analog beamforming systems, can only transmit in one or a few beams simultaneously. In such systems, multiple beams may have to be scanned through in time domain to provide coverage to all the UEs attached to the base station. Thus, as illustrated in FIG. 1D, only one antenna beam 34-36 can be transmitted at a time, for example, due to an analog beamforming implementation. As a result, different beams 34-36 and corresponding addressed UEs 22-24 may be time-multiplexed using time intervals at times t=0, t=1, and t=2, as shown. The antenna beams 34-36 may be beamformed, but may be steered in a manner similar to the beams 30-32 in FIG. 1C to provide the coverage range necessary to cover all the UEs 22-24 attached to the base station 20.
It is noted that, for ease of discussion, the terms “analog beamforming,” “beamforming,” “narrow beamforming,” and other terms of similar import may be used interchangeably herein.
Beamforming systems may also have a calibration mismatch between the transmit (Tx) and receive (Rx) sides of an antenna array. On the other hand, some beamforming systems may even have separate antenna arrays for transmission and reception, such that beamforming-related directional information regarding a beam received in the Uplink (UL) may not be applied to a beam transmitting in the Downlink (DL). It is noted here that the terms Uplink and Downlink are used in their conventional sense: a transmission in the UL refers to a UE's transmission to a base station, whereas a transmission in the DL refers to a base station's transmission to a UE. In the context of beam-forming, FIG. 1E shows an example where two separate antenna arrays—a Tx array 38 and an Rx array 40—may form part of a base station's antenna system. It is seen from the illustration in FIG. 1E that the most suitable DL beam 42 for a UE 43 is different from the corresponding UL beam 44 due to separate Tx and Rx arrays at the base station. The DL beam 42 may be “most suitable” or “good enough” for the UE 43 in the sense that the beam 42 may allow the base station to establish and maintain transmissions to the UE 43. On the other hand, the UL beam 44 may be “most suitable” or “good enough” in the sense that the beam 44 may allow the base station to receive transmissions from the UE 43. However, in contrast to the configurations in FIGS. 1A-1D, the configuration in FIG. 1E uses two different beams 42, 44—one for the DL and the other for the UL, respectively.
In the time-multiplexed beam-forming implementation of FIG. 1D or the “mismatched” beams of FIG. 1E, a corresponding UE may have to first “attach” to the cell or base station before the UE can transmit/receive user data to/from the base station. Before a UE can “attach” to a cell, the UE may need to acquire system information of the corresponding cell when the UE tries to initially access the cell. In a Third Generation Partnership Project's (3GPP) Long-Term Evolution (LTE) cellular network, a random-access procedure is a key function that may need to be carried out to enable a UE to attach to the respective cell—regardless of whether the UE is attaching to the cell in a synchronized or non-synchronized mode.
FIG. 2 depicts an exemplary messaging flow 46 for a random-access procedure in a Fourth Generation (4G) LTE cellular network. For ease of discussion, the messaging flow 46 is shown with reference to an eNB 48 and a UE 50. The UE 50 may be similar to any of the UEs 22-24 and 43 in FIGS. 1A-1E. Similarly, the eNB 48 may be similar to the base station 20 in FIGS. 1A-1D or may constitute the antenna arrays 38, 40 of FIG. 1E. As noted before, for ease of discussion, the reference numeral “48” may be interchangeably used herein to refer to the eNB 48 or its corresponding cell (not shown). Cell search is part of the random-access procedure by which a UE may acquire time and frequency synchronization with a cell—more particularly, with a specific base station in the cell—and may detect the physical layer Cell ID of that cell. As shown at block 52 in FIG. 2, the eNB 48 may broadcast two special signals—a Primary Synchronization Sequence (PSS) and a Secondary Synchronization Sequence (SSS)—in an Orthogonal Frequency-Division Multiplex (OFDM) symbol. These broadcast signals may be received by all UEs operating in the cell 48, including the UE 50. The detection of these signals allows the UE 50 to perform time and frequency synchronization—indicated in block 52 as “subframe and radio frame synchronization”—with the eNB 48 and to acquire useful system parameters such as cell identity (physical cell ID). In LTE, the PSS and SSS synchronization signals may be transmitted twice per 10 ms radio frame. As is known, a 10 ms radio frame in LTE constitutes ten (10) “subframes” of 1 ms each. Thus, in LTE, a Transmission Time Interval (TTI) of 1 ms is referred to as a “subframe.” The PSS signal may be the same for any given cell in every subframe in which it is transmitted.
As shown at block 54 in FIG. 2, the eNB 48 may also transmit Physical Broadcast Channel (PBCH) and Physical Downlink Shared Channel (PDSCH) signals in the corresponding cell. The PBCH may provide such basic information as the downlink system bandwidth, which may be essential for the initial access of the cell. However, the PBCH is designed to be detectable without prior knowledge of system bandwidth and to be accessible at the cell edge as well. Through the PBCH, the UE 50 may be requested to also receive the PDSCH to obtain important System Information (SI). The time interval between successive transmissions of the PBCH may be 40 ms. The PDSCH is the main data-bearing channel, which is allocated to users/UEs in the cell on a dynamic and opportunistic basis. In addition to sending user data to a UE, the PDSCH is also used to transmit general scheduling information and other broadcast information not transmitted on the PBCH such as, for example, the SI including System Information Blocks (SIBs). In LTE, SIBs may be scheduled by Physical Downlink Control Channel (PDCCH). This general scheduling information may not be UE-specific. However, any UE-specific scheduling information such as, for example, how to decode the SIBs, may be transmitted by the eNB 48 on an Enhanced PDCCH (ePDCCH) after random access is completed.
Upon receiving the synchronization signals at block 52 and the system information at block 54, the UE 50 may attempt to access the network and initiate the random-access procedure by transmitting a random-access preamble in the uplink on a Physical Random-Access Channel (PRACH), as indicated at block 56. The preamble allows the eNB 48 to estimate the timing-advance necessary for the UE 50. This timing advance is then communicated to the UE 50 a Random-Access Response (RAR) message at block 58 (discussed below). Only after receiving the RAR can the UE 50 synchronize its timing with the eNB 48 so as to “attach” to the eNB 48 or “camp” on the cell. The UE's 50 messaging at block 56 may be referred to herein as “Message1” or “Msg1.” Upon receiving the preamble and detecting the UE's random-access attempt, the base station 48 may respond in the downlink by transmitting an RAR message on the PDSCH, as indicated at block 58. In the discussion herein, the terms “RAR message” and “Message2” (or “Msg2”) may be interchangeably used to refer to the eNB's 48 response to the preamble-carrying Msg1 at block 56 during the random-access procedure. The random-access response at block 58 may be referred to in the relevant literature as a “RAR grant”, which may be a 20-bit uplink scheduling grant for the UE 50 to continue the random-access procedure by transmitting a subsequent message—referred to herein as “Message3” or “Msg3”—in the uplink. The content of the RAR grant is defined in section 6.2 of the 3GPP Technical Specification (TS) 36.213, version 12.5.0 (March 2015), titled “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Layer Procedures (Release 12).” The discussion in the section 6.2 of the 3GPP TS 36.213 is incorporated herein by reference in its entirety.
After adjusting its uplink timing, if necessary, based on the UL grant in Msg2, the UE 50 may transmit the Msg3 to the eNB 48 on a Physical Uplink Shared Channel (PUSCH) and provide its terminal identification (terminal ID) in Msg3, as indicated at block 60. Like the PDSCH, the PUSCH also carries user data. Furthermore, the UEs may be scheduled on the PUSCH and PDSCH in 1 ms scheduling interval—that is, in a time interval equal to a subframe.
Upon receiving UE's response (Msg3) to the RAR message (Msg2), the eNB 48 may determine if contention resolution is required such as, for example, when the eNB 48 receives two random-access preambles from two different UEs, but with the same value at the same time. The eNB 48 may resolve the contention and select one of the UEs. As noted at block 62, the eNB 48 may then send a “Message4” or “Msg4” to the selected UE—here, the UE 50—in the PDSCH.
At block 64, the UE 50 is “attached” to the eNB 48 and may establish a bi-directional communication session with the eNB 48 using the PDSCH and PUSCH to transfer user data to/from UE's user. The user can then use the UE 50 to carry out voice calls, data sessions, web browsing, and the like, using the cellular network of the eNB 48.